This invention relates to an optical disk apparatus and, more particularly, to an optical disk apparatus in which data can be read from an optical disk accurately even in a case where resolution has declined owing to an increase in density or a change in recording conditions.
Further, the present invention is related to an optical disk apparatus and, more particularly, to an optical disk apparatus in which data is recorded by the absence or presence of marks and data is demodulated from a reproduced signal obtained by reading the marks from an optical disk on which mark edges have been recorded as "1"s.
In recent years an optical disk apparatus has been developed as an external storage device for a computer and the apparatus has been put into practical use. In an optical disk apparatus, a semiconductor laser is narrowed down to a very small spot on the wavelength order and data is recorded on a medium. A major feature of such an apparatus is the possibility of high-capacity recording. In particular, it is foreseen that an optical disk apparatus for five-inch and 3.5-inch disks standardized by the ISO specifications will find wide use in applications ranging from high-performance work stations to devices on the individual user level.
The available optical disks include writable magneto-optical disks which comprise a substrate and an amorphous, magnetic thin film such as TbFeCo deposited on the substrate. Such a disk has a property in which the retentiveness necessary for magnetic reversal of the magnetic film diminishes in conformity with a rise in temperature (retentiveness is zero at the Curie point). More specifically, recording and erasure are performed by irradiating the disk with a laser beam to raise the temperature of the disk medium to the vicinity of 200.degree. C., thereby weakening retentiveness, applying a weak magnetic field under this condition and controlling the direction of magnetization. Accordingly, as illustrated in FIG. 20A, an upwardly directed magnetic field is applied by a writing coil 6 under a condition in which the direction of magnetization of a magnetic film 5 is pointed downward. When a portion at which the direction of magnetization is desired to be changed is irradiated with a laser beam LB via an objective lens OL, as shown in FIG. 20B, the direction of magnetization of this portion reverses, i.e., is pointed upward. This makes it possible to record information. When information is read, the magnetic film 5 is irradiated with a laser beam LB having a plane of polarization along the y axis, as illustrated in FIGS. 20C, 20D. When this is done, reflected light LBO, in which the plane of polarization has been rotated by .theta..sub.k in the clockwise direction owing to the magnetic Kerr effect, is obtained in the portion where magnetization is downwardly directed. In the portion where magnetization is upwardly directed, reflected light LB1, in which the plane of polarization has been rotated by .theta..sub.k in the counter-clockwise direction owing to the magnetic Kerr effect, is obtained. Accordingly, the direction of magnetization, namely information, can be read by detecting the state of polarization of reflected light.
Such magneto-optical disks include (1) a full RAM disk the entire surface of which is writable, (2) a partial ROM disk having a writable area (a RAM area) and a read-only area (a ROM area), and (3) a full ROM disk the entire surface of which is a ROM area.
Constitution of magneto-optical disks
FIGS. 21A, 21B are diagrams for describing the constitution of, say, a 3.5-inch magneto-optical disk, in which FIG. 21A is a general plan view and FIG. 21B a partial sectional view. A magneto-optical disk 11 has tracks in the form of concentric circles or a spiral. All of the tracks are partitioned into 25 sectors. Each sector is composed of, say, 725 bytes, and is provided at its head with an address field AF (ID area). The address field AF is followed by a data field DF (MO area, which stands for "magneto-optical area"). Recorded in the address field AF are a sector mark and address information such as a track address, sector address and preamble for reproducing a synchronizing signal. Stored in the data field DF are a VFO pattern for clock extraction, a synch byte SYNC for phase synchronization, and data DATA.
As shown in FIG. 21B, the magneto-optical disk 11 is constructed by depositing a recording layer (recording film) MGF on a transparent plastic layer (substrate) PLS, and forming a protective layer PRF on the recording film MGF. An address field AF (ID area) is preformatted by pits PT formed in advance by stamping.
System configuration utilizing magneto-optical disk medium
FIG. 22 is a diagram showing the configuration of a system which utilizes a magneto-optical disk medium. The system includes the magneto-optical disk 11, a magneto-optical disk drive 21, a host system 31 (the main body of a computer), and a data input unit (control panel) 41, which has a keyboard 41a and a mouse 41b. Numeral 51 denotes a display unit such as a CRT or liquid-crystal display, and 61 represents a printer. A hard disk device and floppy disk device are provided as necessary.
FIG. 23 is a diagram showing the electrical configuration of the system, in which portions identical with those shown in FIG. 22 are designated by like reference characters. Numeral 21 denotes the magneto-optical disk drive, 22 a hard disk drive, 31 the host system, 71a.about.71b I/O controllers and 72 an SCSI (small computer system interface) bus. An SCSI is an interface that connects the main body of a computer with an external storage device. The specifications of an SCSI are stipulated by the American National Standard Institute (ANSI). The SCSI bus 72 is composed of a data bus, which comprises eight bits and a parity bit, and nine control busses, by way of example. Up to a maximum of eight SCSI devices (a host computer, a disk drive controller, etc.) can be connected to the SCSI bus, and each device connected has an identification number, referred to as an "ID" (identifier), of from 0 to 7. In FIG. 23, identifiers ID0.about.ID1 are allocated to the I/O controllers 71a, 71b, respectively, and ID7 is allocated to the host computer 31. Though one optical disk drive 21 and one hard disk drive 22 are connected to the I/O controllers 71a, 71b, more than two drives can be connected.
The host system 31 includes a central processor 31a, a memory 31b, a DMA controller 31c, a host adapter 31d and I/O controllers 71c, 71d, all of which are connected to a host bus 31e. The host system 31 uses a floppy disk drive 23, which is connected to the I/O controller 71c. The host system further includes the control panel 41, the display device 51 and the printer 61, all of which are connected to an I/O controller 71d.
The host system 31 and the I/O controllers 71a, 71b are interconnected by an SCSI interface, and the I/O controllers 71a, 71b are connected to the respective drives 21, 22 by ESDIs (enhanced small device interfaces), by way of example. In this system the magneto-optical disk drive 21 and the hard disk drive 22 are separated from the host bus 31e, the SCSI bus 72 is provided separately of the host bus, the I/O controllers 71a, 71b for the drives are connected to this SCSI bus, and the drives 21, 22 are controlled by the I/O controllers 71a, 71b, respectively, to lighten the burden upon the host bus.
Basic construction of magneto-optical head
FIG. 24 is a diagram showing the basic construction of a magneto-optical head used in the magneto-optical disk drive 21. The magneto-optical head includes a semiconductor laser 21.sub.1, a collimator lens 21.sub.2, a true-circle correcting prism 21.sub.3, a beam splitter 21.sub.4 for transmitting light from the semiconductor laser 21.sub.1 and reflecting light, which has been reflected by the disk (not shown), toward a signal detection side, a reflecting mirror 21.sub.5 for introducing light to the disk, and a two-dimensional actuator 21.sub.6 having an objective lens, a tracking coil and focusing coil for finely adjusting the objective lens in tracking and focusing directions, and a biasing coil for applying an external magnetic field when data is written. The head further includes a reflecting mirror 21.sub.7 for introducing reflected light to a data detection side, a halfwave plate 21.sub.8 for rotating the plane of polarization of incident light by 45.degree. and establishing a ratio of 1:1 between the amount of light transmitted and the amount of light reflected by a polarization beam splitter, which is a subsequent stage. The head further includes a converging lens 21.sub.9, the aforementioned polarization beam splitter 21.sub.10, a P-wave component detector 21.sub.11, and an S-wave component detector 21.sub.12.
Fundamentals of reading MO-area information
The polarization beam splitter 21.sub.10 transmits light which is parallel to the plane of incidence (this light is the P-wave component) and reflects light which is perpendicular to the plane of incidence (this light is the S-wave component). Accordingly, the state of polarization of incident light can be detected as a change in the amount of light transmitted and a change in the amount of light reflected. More specifically, the plane of polarization of returning light is rotated by .theta..sub.k in the clockwise or counter-clockwise direction, as shown in FIG. 25A, by the magnetic Kerr effect in dependence upon the direction of magnetization (information bits "0", "1") in the reader which reads the MO area, and the plane of polarization is rotated by 45.degree. by the halfwave plate 21.sub.8. Consequently, with regard to the P-wave component (transmitted light) and S-wave component (reflected light) outputted by the polarization beam splitter 21.sub.10, the P-wave component becomes larger than the S-wave component when the information is "1" and smaller than the S-wave component when the information is "0", as shown in FIG. 25B. Accordingly, the P-wave component detector 21.sub.11 outputs a signal RDS1 shown in FIG. 25C, the S-wave component detector 21.sub.12 outputs a signal RDS2 (the polarity of which is the opposite of that of signal RDS1) shown in FIG. 25C. If these signals RDS1, RDS2 are fed into a differential amplifier, a reproduced signal RDS, from which noise of the same phase has been removed, is obtained.
Magneto-optical disk drive
FIG. 26 is a diagram showing the construction of the magneto-optical disk drive 21. Numeral 21a denotes the magneto-optical head shown in FIG. 24. The head includes the semiconductor laser 21.sub.1, the P-wave component detector 21.sub.11, the S-wave component detector 21.sub.12 and an objective lens OL. Numeral 21b denotes a controller constituted by a microcomputer. The controller 21b performs overall control of the magneto-optical disk drive in accordance with commands from the host system 31 (see FIG. 22), e.g., positioning control of the magneto-optical head, control for reading and recording of data, etc. Numeral 21c denotes a head-access control circuit for positioning the magneto-optical head 21a at a prescribed position in accordance with an command from the controller, 21d a data recording circuit for recording data on the magneto-optical disk, and 21e a data reproducing circuit for reproducing data that has been recorded on the magneto-optical disk.
Upon receiving a data-read command from the host (the main unit 31 of the system), the controller 21b performs in such a manner that the magneto-optical head 21a is positioned at the commanded address by the head-access control circuit 21c and made to read the recorded signal. The magneto-optical head 21a inputs the read signal to the data reproducing circuit 21e. The latter reproduces data from the signals which enter from the detectors and inputs the reproduced data to the controller 21b. The latter inputs this data to the host.
Upon receiving a data-write command from the host, the controller 21b performs in such a manner that the magneto-optical head 21a is positioned at the commanded address by the head-access control circuit 21c and the semiconductor laser 21.sub.1 is turned on and off based on the write data to write the data on the magneto-optical disk. In the case of the 5-inch or 3.5-inch recording format standardized in accordance with the ISO, the recorded data is encoded by RLL (2,7) encoding and the encoded data is written in the MO area. According to RLL (2,7) encoding, the number of "0"s between "1" bits after encoding varies from two to seven. The input data and encoded data are related as shown below. It should be noted that RLL is the abbreviation of "run-length limited".
______________________________________ Input Data Encoded Data ______________________________________ 10 0100 010 100100 0010 00100100 11 1000 011 101000 0011 00001000 000 000100 ______________________________________
Encoding methods include RLL (1,7) encoding in addition to the aforementioned RLL (2,7) encoding. According to RLL (1,7) encoding, the number of "0"s between "1" bits after encoding varies from one to seven.
Data reproducing circuit
FIG. 27 is a diagram for describing a data reproduction method. Data to be recorded is encoded to data having a format (the aforementioned RLL 2,7 code) suited to the recording characteristics of the optical disk. In actual recording, recorded pits (the black circles in FIG. 27) are made to correspond to the "1" bits of the encoded data. The size of a recorded pit is on the order of the wavelength of the semiconductor laser. In the case of a presently available 3.5-inch medium according to ISO specifications, the bit-cell spacing of encoded data is smallest at the inner circumference, or about 0.75 .mu.m.
The reproduction of data is performed by detecting a change in amount of light when a recorded pit is scanned by the semiconductor laser. The actual waveform of the reproduced signal RDS possesses peaks at the points in time at which the marks (recording pits) are present. Accordingly, reproduction of data can be performed by detecting the peak points of the reproduced signal RDS. Specifically, a gate signal GTS is created by differentiating the reproduced signal and detecting the fact that the level of the differentiated signal (DFS) is greater than a certain value. Further, the differentiated signal DFS is binarized at a zero level to create a zero-cross signal ZCS which crosses zero level at the peak points of the reproduced signal RDS. When the gate signal GTS is at the high level and the zero-cross signal ZCS decays, a reproduced-data signal DT having a prescribed duration is outputted.
FIG. 28 is a diagram showing the construction of the data reproducing circuit 21e. The circuit includes an amplifier 21e-1 for amplifying the reproduced signal RDS, a low-pass filter 21e-2, a differentiating circuit 21e-3 for differentiating the reproduced signal, a comparator 21e-4, to which the differentiated signal DFS is applied, for comparing this signal with a set value, thereby outputting the gate signal GTS, a comparator 21e-5 for binarizing the differentiated signal DFS at the zero level and outputting the zero-cross signal ZCS, a flip-flop 21e-6 for outputting the reproduced-data signal when the gate signal GTS is at the high level and the zero-cross signal ZCS decays, a delay unit 21e-7 for setting a prescribed duration W, a PLL circuit 21e-8 for extracting a clock contained in the reproduced data, and a data separator 21e-9 for outputting data in synchronism with the extracted clock.
The PLL circuit 21e-8 includes a phase comparator PHS for outputting a phase-difference signal indicating the phase difference between the reproduced data DT and the output of VFO, a charge pump CPMP for outputting a voltage which conforms to the phase difference, a low-pass loop filter LPFL, and a voltage frequency oscillator VFO for outputting a signal whose frequency conforms to the output voltage of the filter LPFL.
The reproduced data DT has a frequency which differs from that at the time of recording owing to fluctuation in the rotation of the spindle motor which rotates the disk and the disks eccentricities. The latter relates to the fact that the center of the disk shifts from the center of rotation. Because of the difference in frequency, the clock synchronized to the reproduced data is extracted from this data by the PLL circuit 21e-8. The "1", "0" data is discriminated by the data separator 21e-9 on the basis of the extracted clock.
In the conventional method of reproduction, resolution (which corresponds to V2/V1 in FIG. 27) declines and the gate signal can no longer be produced owing to a fluctuation in the sensitivity of the medium or in the recording power or as the result of an increase in density. For example, in the case of a 3.5-inch, 128-megabyte optical disk, minimum bit spacing is 1.5 .mu.m and V2/V1 is 50.about.60%. When V2/V1 is on the order of 50.about.60%, approximately the same differentiated-signal amplitude can be obtained irrespective of whether the peaks of the reproduced signal RDS are isolated or clustered together. However, if the storage capacity is 230 megabytes according to next-generation ISO standards, recording density increases on the order of 20%. As a consequence, the reproduced signal RDS becomes as shown in (a) of FIG. 29, where it is seen that resolution V2/V1 diminishes. As a result, the differentiated signal DFS becomes as shown in (b) of FIG. 29. Even if the gate signal GTS is generated by slicing the differentiated signal DFS at a prescribed level V.sub.s, the gate signal GTS will not be accurate at portions where the peaks are close together. Undetected peak portions are produced and a data reading error occurs [see (c) in FIG. 29].
Accordingly, consideration has been given to providing an equalizing circuit in back of the low-pass filter 21e-2 and emphasizing high-frequency components by means of this circuit to enlarge the amplitude V2 of the reproduced signal at a peak cluster, thereby increasing resolution. When high-frequency components are emphasized by the equalizing circuit, however, the reproduced signal RDS develops an undershoot US, as shown in FIG. 30(a), at a portion of low density. This means that when the reproduced signal is differentiated, the portion corresponding to overshoot exceeds the slice level V.sub.S, as illustrated in (b) of FIG. 30, and an erroneous gate pulse GTS is generated. The problem that arises as a result is the generation of an erroneous reproduced-data pulse. Further, in the method of raising resolution by waveform equalization, high-frequency noise is emphasized at the same time and hence there is a decline in S/N ratio, which is a measure of signal quality. Another problem is that the zero-cross signal occurs at the wrong point.
What is meaningful in the reproduced signal RDS is peak point. According, it will suffice to detect the peak level of the reproduced signal by suitable means and obtain a slice level VS for creating a gate signal GTS using the peak level as a reference. However, owing to a transient at the beginning of a sector, which is the data management area of an optical disk, as well as a variation in the envelope of the reproduced signal which accompanies a fluctuation in reflection by the medium, a mechanism for detecting the peak envelope is required. FIG. 31 illustrates the manner in which transience occurs. The transient occurs due to the fact that the reproduction system is AC-coupled and DC components are lost. The band of this AC coupling is set to be sufficiently low (1/100.about.1/50 of signal frequency) in order to prevent the reproduced signal RDS from becoming distorted. At a fixed slice level, therefore, data over a considerable area is lost from the moment the data starts.
FIG. 32 shows one example for deciding slice level using a peak hold circuit. Numeral 81 denotes a peak hold circuit comprising a diode and a capacitor, and numeral 82 designates a slice-level deciding circuit. The capacitor C accumulates the peak values of the reproduced signal RDS. More specifically, when the reproduced signal RDS exceeds the terminal voltage of the capacitor, the diode turns on and the capacitor is charged. When the reproduced signal RDS falls below the peak value, the diode is cut off and the accumulates electric charge is discharged via the resistor R (time constant .tau.=CR). As a result, the terminal voltage of the capacitor C varies while following up the peak value and enters the slice-level deciding circuit 82 via a buffer amplifier BA. The slice-level deciding circuit 82 decides the slice level VS, which produces the gate signal, on the basis of the peak value of the reproduced signal RDS. In order for the peak value of the reproduced signal RDS to be held accurately by the peak hold circuit 81, a high-speed buffer amplifier is required. More specifically, a follow-up characteristic on the order of nanoseconds is needed. Another problem is the precision of the buffer amplifier, which is required in order to reduce leakage from the capacitor C holding the peak value. Furthermore, a function for canceling out a decline in the forward voltage of the diode is necessary. Thus, employing the peak hold circuit involves many shortcomings in actual application.
Accordingly, there is demand for an optical-disk reproduction apparatus in which even if there is a decline in resolution owing to an increase in density or a change in recording conditions, data can be read from the optical disk accurately and the adverse effects caused by the transient at the start of data and by wave form fluctuation, which effects are ascribable to the AC coupling of the reproduction system, can be prevented.
FIG. 33 is a block diagram illustrating an information reproducing apparatus proposed in Japanese Patent Application Laid-Open (KOKAI) No. 3-102677, and FIG. 34 is a waveform diagram showing various waveforms useful in describing the operation of this apparatus. The apparatus includes an amplifier 91 for the reproduced signal RDS, a low-pass filter 92, a differentiating zero-cross detecting circuit 93 for differentiating the reproduced signal RDS and generating a zero-cross signal ZCS when the differentiated signal DS crosses the zero level, a clamping circuit 94 for clamping the lower-limit level of the reproduced signal RDS, a gate signal generating circuit 95 for outputting, as the gate signal GTS, a binarized signal obtained by slicing the output signal Vout of the clamping circuit 94 at a prescribed level L.sub.s, and a reproduced digital-signal generating unit 96 for generating a reproduced digital signal DT which reverses when the zero-cross signal ZCS decays while the gate signal GTS is at the high level and reverses again when the gate signal GTS assumes the low level after generation of the zero-cross signal.
The clamping circuit 94 includes an npn transistor 94a having a base terminal to which a clamp control voltage Vset is supplied and an emitter terminal whose output is the output of the clamping circuit, a resistor 94b connected across the emitter and ground, and a capacitor 94c for cutting DC components. If an input signal Vin is greater than Vcc, the transistor 94a turns off and the input signal Vin is outputted as vout. If the input signal Vin is less than Vcc, on the other hand, the transistor 94a turns on and the biasing voltage Vcc is outputted. Thus, the clamping circuit 94 outputs a signal whose lower-limit level is clamped to Vcc.
In accordance with this clamping circuit, the lower-limit level of the reproduced signal RDS corresponding to modulation data "0" (no mark) can be clamped at a fixed voltage, as shown at (e) in FIG. 34. That is, even if the resolution of the reproduced signal RDS is low, the resolution can be increased equivalently by passing the signal through the clamping circuit 94 and it is possible to create the gate signal GTS in such a manner that the gate signal reliably attains the high level in a range which includes the peak points of the reproduced signal (the zero-cross points of the differentiated signal). Data can thus be reproduced correctly in the unit 96 which generates the reproduced digital signal.
A draw back of transistor clamping (inclusive of diode clamping), is that the time for the transistor to make the transition from on to off is long owing to the effects of the electric charge accumulated at the base. As a consequence, a time shift of from several nanoseconds to several microseconds develops between the reproduced signal RDS and the clamped output signal Vout, and a time shift Td develops in the gate signal GTS as well. The time shift developed by the gate signal GTS causes problems when density is increased (when data is transferred at high speed). Further, the proposed clamping circuit clamps the lower-limit level. Therefore, when the reproduced signal RDS picks up noise NS caused by defects in the disk, the peak value of the noise exceeds the slice level L.sub.s and erroneous reading results, as illustrated in FIG. 36.
Further, the prior art employs a recording method in which the center of a mark is adopted as "1". The reason for this is that if the reproduced signal RDS is differentiated and zero-cross detected, in the manner set forth above, then the position (timing) of a "1" can be clearly discriminated. However, the demand for greater capacity never ceases and a higher recording density is always being sought. Accordingly, a method has been proposed in which both edges of a mark are recorded as "1"s. According to this method, as depicted in FIG. 37, the recording of marks is started at odd-numbered "1"s and marks are ended at even-numbered "1"s, as a result of which recording frequency declines. This means that recording can be performed at a density twice that achieved with the recording method that adopts the center of a mark as "1". It should be noted that FIG. 37 illustrates the reproduced signal RDS in a case where a laser beam has been narrowed down by an objective lens OL and "1" data has been read at mark edges by means of the laser beam.
However, a precise proposal has not been made with regard to reproducing means for reproducing data accurately from an optical disk on which mark edges have been recorded as "1"s.
Accordingly, there is demand for an apparatus capable of accurately reproducing data recorded by such a recording method. Fundamentally, it will suffice to adopt a level intermediate the peak and bottom of the reproduced signal as the slice level, slice the reproduced signal at this slice level and discriminate marks from spaces. In actuality, however, the following problem arises:
The light signal to be detected is extremely weak and AC coupling is necessary in order to amplify this signal to a practical amplitude (on the order of one volt). When a reproduction system is AC coupled and DC components are lost, the transient occurs (see FIG. 31), as mentioned earlier. Consequently, with the method of slicing the reproduced signal at the intermediate level Vs of the peak Vp and bottom Vb of the reproduced signal RDS in the steady state an then discriminating between marks and spaces, data over a considerable area is lost from the moment the data starts. In addition, depending upon the pattern of the code data, the peak level or bottom level fluctuates and, hence, so does the intermediate level. For example, the intermediate level is high at portions where "1"s and "0"s alternate at high frequency and is low at portions where "1"s and "0"s alternate at low frequency. In such case the data cannot be reproduced accurately. Though consideration has been given to performing differentiation twice so that there will be no influence from AC coupling, the noise component is enlarged and it is difficult to distinguish data only.